Field of the Invention
The subject matter disclosed herein concerns improvements to gas turbine systems used in mechanical drive applications. In particular, but not exclusively, the disclosure concerns gas turbine systems for driving compressors, e.g., compressors for refrigerant fluids in liquefied natural gas facilities, compressors for compressing gas in pipeline transportation etc.
The disclosure further concerns improvements in methods for operating a system comprising a gas turbine and a load, e.g., a compressor for LNG, or for oil and gas applications, a pump, or other rotary equipment.
The disclosure further concerns a system for storing pressure energy in a pipeline for gas.
Description of the Related Art
Liquefied Natural Gas (LNG) results from a liquefaction process, in which the natural gas is cooled using one or more refrigeration cycles in a cascade arrangement, until it becomes liquid. Natural gas is often liquefied for storage or transportation purposes, e.g., if pipeline transportation is not possible or economically unfeasible.
Cooling of the natural gas is performed using closed or opened refrigeration cycles. A refrigerant is processed in a compressor or compressors, condensed and expanded. The expanded, chilled refrigerant is used to remove heat from the natural gas flowing in a heat exchanger.
On the contrary, when possible or economically feasible, for transporting gas a pipeline transportation is generally used. To maintain the gas under pressure in the pipeline, one or more compressors are arranged along the pipeline.
Refrigerant compressors in LNG, compressors for pipeline applications or other rotary equipment for applications in the oil and gas industry, are often driven by gas turbines. The gas turbine power availability (i.e., the power available on the turbine power shaft) is dependent upon ambient conditions, e.g., air temperature, and other factors, such as ageing. The turbine power availability increases with decreasing temperatures and, conversely, decreases with increasing temperatures. This causes power availability fluctuations both in the 24 hours as well as during the year, due to daily and seasonal temperature fluctuations.
It has been suggested to provide an electric motor in combination with a gas turbine (e.g., a heavy duty gas turbine or an aero-gas turbine) to drive a load, comprised of, e.g., one or more compressors. The electric motor is operated to supplement mechanical power to the load, to maintain the overall mechanical power on the load shaft constant, when power availability of the turbine decreases and/or to increase the total mechanical power used to drive the load. This function of the electric motor is referred to as helper duty. Another electric motor or, alternatively a pneumatic motor, is usually used also as a starter motor, to accelerate the gas turbine from zero to the rated speed.
When an excess mechanical power is generated by the turbine, e.g., if the ambient temperature drops below the design temperature and consequent increase in power availability of the turbine, or mechanical load required by the compressor drops, the excessive mechanical power generated by the gas turbine is converted into electric power, using the electric helper motor as a generator.
FIG. 1 schematically illustrates a system comprising a gas turbine arranged for mechanical drive applications, e.g., for driving a compressor or compressor train. The system 101 comprises a heavy duty gas turbine 103. The gas turbine is in turn comprised of a gas generator 105 and a power turbine 107. The gas generator 105 is comprised of a compressor 109 and a high-pressure turbine 111. The gas generator 105 comprises a gas generator rotor including the rotor 109R of the compressor 109 and the rotor 111R of the high-pressure turbine 111. The rotor 109R of the compressor 109 and the rotor 111R of the high-pressure turbine 111 are mounted on a common shaft and together form a gas generator rotor.
The compressor 109 compresses ambient air, which is delivered to a combustion chamber or combustor 113, where the compressed air is mixed with a liquid or gaseous fuel and the fuel/air mixture is ignited to generate combustion gas. The high-temperature and high-pressure pressure combustion gas is partly expanded in the high-pressure turbine 111. Mechanical power generated by the gas expansion in the high-pressure turbine 111 is used to drive the compressor 109.
Hot and partly expanded gas exiting the high-pressure turbine 111 flows through the power turbine or low-pressure turbine 107. The combustion gas expands in the power turbine 107 to generate mechanical power made available on a load coupling shaft 117. The power available on the load coupling shaft 117 is used to drive into rotation a load globally labeled 121. The load 121 can comprise a compressor or a compressor train, as an example, arranged in a pipeline 118 for transporting gas to users 120.
The rotor of the power turbine 107 is mechanically separated from, i.e. not torsionally coupled to, the gas generator rotor formed by the compressor rotor 109R and the high-pressure turbine rotor 111R.
The gas generator rotor is connected to a starter motor 124. As an example, this starter motor can be an electric motor connected, through a shaft 106, to the gas generator rotor.
Reference number 123 indicates a reversible electric machine, operating as a helper/generator and arranged at the end of the string comprising the gas turbine 103 and the compressor 121, opposite the starter motor 124. The electric machine 123, when operating as helper, converts electric power into mechanical power to drive the load 117 in combination with the gas turbine 103, for example when the power available from the gas turbine 103 drops, for instance due to increasing environment temperature. When the electric machine 123 is acting as generator, the reversible electric machine can convert available mechanical power, produced by the turbine, into electric power. The electric power can be delivered to an electric power distribution grid.
The system 101 is complex, has a modest operating flexibility, and incurs in some disadvantages. As an example, in a pipeline transportation system, when the gas turbine goes under maintenance or fails, the users 120 cannot extract gas without incurring in a significant pressure drop in the pipeline 118.